One of the important issues in rolling operations of metal plate and sheet materials is to make the elongation rate of the rolled material equal at the work side and the drive side. Hereinafter, for simplification of expression, the work side and the drive side will be referred to as the “left” and “right”. If the elongation of the rolled material becomes uneven at the left and right, camber and plate thickness wedges, that is, defects in the flat shape and dimensional precision of the rolled material, will occur. Not only that, running trouble such as meandering and drawing will sometimes occur.
As work means for making the left and right elongation rates in rolling of a rolled material equal, eliminating the difference in the roll gap positions of the rolling mill at the work side and the drive side, that is, a left-right asymmetric control of roll gap (work side-drive side asymmetric control of roll gap), is used. Usually, a left-right asymmetric control of roll gap is performed by establishing proper settings before rolling, ensuring suitable operation during rolling, and having the operator carefully observe the rolling operation during work, but it cannot be said that the above-mentioned camber and plate thickness wedge quality defects and running trouble have been able to be sufficiently controlled.
In view of the above issues, PLT 1 discloses the art of performing a left-right asymmetric control of roll gap based on the ratio of the sum of the difference of the load cell loads of the work side and drive side of the rolling mill. Further, PLT 2 discloses the art of performing a left-right asymmetric control of roll gap by directly detecting the offset from the rolled material at the rolling mill entrance side, that is, the meandering.
The arts disclosed in the above PLT 1 and PLT 2 for reducing to zero the difference in elongations of the rolled material at the work side and the drive side illustrated here all aim at optimizing left-right asymmetric control of roll gap as means of control, but in each art, a difference arises in the elongation rate of the rolled material at the work side and the drive side. These are arts for control by the left-right asymmetric control of roll gap and do not optimize the setting of the left-right asymmetric control of roll gap before start of rolling.
One of the most important factors in left-right asymmetric control of roll gap control before the start of rolling is the zero point adjustment of the roll gap position. Usually, in a flat product rolling mill, after rolls are exchanged, zero point adjustment of the roll gap position (hereinafter, also called “roll gap zeroing” or simple “zeroing”) is performed. In this method, in the roll turning state, the reduction apparatus is operated to set the kiss roll state then the point of time when the measurement value of the rolling load matches a predetermined zero point adjustment load (setting preset as 15% to 85% of rated load) is made the zero point of the roll gap position. This is often employed after installing new rolls etc.
At this time, the difference between the left and right roll gap positions is usually eliminated, that is, the zero point of left-right asymmetric control of roll gap is also simultaneously adjusted. Regarding the zero point adjustment of left-right asymmetric control of roll gap as well, at the time of the kiss roll state, the measurement values of the rolling load at the work side and the drive side are adjusted to match the predetermined zero point adjustment loads. Note that the “kiss roll state” is the state with no rolled material present where the upper and lower work rolls are made to contact each other and a load is given between the rolls.
PLT 3 discloses a method of zero adjustment which maintains a kiss roll state until the sum of the measurement loads of the work side and the drive side becomes a predetermined value and, while maintaining the sum of the loads at a predetermined value, performs a left-right asymmetric control of roll gap so that the left and right load measurement values become the same.
Now, between work rolls and backup rolls or, in the kiss roll state (state where rolls are “kissing”), between upper and lower work rolls, where the rolls cross, a thrust force (force acting in roll axial direction) is generated between the rolls. FIG. 8 shows the state of thrust force occurring in a four-high rolling mill. This thrust force gives extra moment to the rolls. Due to this, the distribution in the roll axial direction of the contact load between rolls changes to balance with the moment. This in the end appears as external disturbance to the difference of the load cells for use for measurement of rolling load of the rolling mill at the work side and the drive side. The cross angle between the rolls need not be intentionally set like with a pair cross rolling mill and also occurs due to the slight clearance presence between the housing and the roll chocks, so it is difficult to control the cross angle to zero.
For this reason, in the art disclosed in PLT 3, when a thrust force is generated, the left-right asymmetric control of roll gap is performed after being affected by external disturbance on the difference of the load cells for use for measurement of rolling load of the rolling mill at the work side and the drive side, so the roll gap position ends up being mistakenly set.
To isolate the effect of the thrust force, for example PLT 4 discloses the method of giving a difference in peripheral speed at the upper and lower work rolls and concentrating the clearance between the housing and the roll chocks at one side to stabilize the chock positions and thereby reduce fluctuation in the thrust force. Further, PLT 5 discloses a method of making the rotation of the work rolls stop and reducing the thrust force at the time of rolling zero adjustment. PLT 6 discloses a method of making the rotation of the work rolls stop at the time of rolling zero adjustment and changing the position in the roll rotation direction by two levels or more to perform rolling zero adjustment, averaging the roll gap positions found by these respective operations, and using that value as the zero point of the roll gap position (initial roll gap position).
Further, PLT 7 discloses the method of measuring the roll axial directional thrust reaction forces acting on all rolls other than the backup rolls and the backup roll reaction forces acting in the rolling direction at the different rolling support positions at the upper and bottom backup rolls, finding one or both of the zero point of the rolling apparatus and the deformation characteristics of the rolling mill, and using these as the basis to set or control the roll gap positions. Further, PLT 8 discloses the method of using the quantity of left-right asymmetric control of roll gap not causing bending before roll replacement as the basis for determining a differential load target value and performing the rolling zero adjustment.
On the other hand, PLT 9 discloses, as a method of control of left-right asymmetric control of roll gap which suppresses the camber of the rolled material, the method of measuring rolling direction forces acting on roll chocks of the work side and the drive side of the work rolls, calculating the difference of the work side and the drive side of the rolling direction forces (also referred to simply as the “difference”), and making this difference become zero by controlling the left and right asymmetric components of the roll opening degrees of the rolling mill.